This invention relates generally to compositions useful as ashless oil-soluble additives for lubricating oils.
Zinc dialkyldithiophosphates (ZDDP) are widely used as lubricant additives. The principal disadvantages of these compounds are that an ash residue is produced by the zinc as the additive is consumed, and that phosphorus is known to affect the efficiency of catalytic converters in motor vehicles, thereby causing emissions problems. An ashless, non-phosphorus alternative to ZDDP would be extremely useful.
A limited number of alkyl-substituted imidazolidinethione compounds is disclosed in A. E. Oberster et al. in Rubber Chem. Technol., vol. 41, page 255 (1968), but there is no suggestion therein that such compounds would be effective as a component in additive compositions for lubricating oils.
The problem addressed by this invention is to find improved phosphorus-free ashless oil-soluble additives for lubricating oils.
The present invention is directed to an oil-soluble lubricant additive composition comprising:
(a) at least one compound of formula I: 
wherein R1, R2, R3 and R4 are independently hydrogen, alkyl, alkenyl, aryl or aralkyl; or R1 and R2, or R3 and R4, combine with the carbon atom to which they are attached to form an alkyl or alkenyl ring; and
(b) an acid having the formula RCO2H, wherein R is C1-C25 alkyl, C2-C25 alkenyl, C2-C25 alkynyl, C5-C25 aryl or C5-C25 aralkyl.
The present invention is further directed to a lubricating oil composition comprising a lubricating oil and from 0.1% to 20% by weight of the lubricant additive composition described above.
An xe2x80x9calkylxe2x80x9d group is a hydrocarbyl group having, unless otherwise specified, from one to twenty-five carbon atoms in a linear, branched or cyclic arrangement. Substitution on alkyl groups of one or more halo, hydroxy, alkoxy, alkanoyl or amido groups is permitted; alkoxy, alkanoyl and amido groups may in turn be substituted by one or more halo substituents. An xe2x80x9calkenylxe2x80x9d group is an xe2x80x9calkylxe2x80x9d group having at least two carbon atoms in which at least one single bond has been replaced with a double bond. An xe2x80x9calkynylxe2x80x9d group is an xe2x80x9calkylxe2x80x9d group having at least two carbon atoms in which at least one single bond has been replaced with a triple bond. An xe2x80x9carylxe2x80x9d group is a substituent derived from an aromatic compound, including heterocyclic aromatic compounds having heteroatoms chosen from among nitrogen, oxygen and sulfur. An aryl group has a total of from five to twenty-five ring atoms, unless specified otherwise, and has one or more rings which are separate or fused. Substitution on aryl groups of one or more halo, alkyl, alkenyl, hydroxy, alkoxy, alkanoyl or amido groups is permitted, with substitution by one or more halo groups being possible on alkyl, alkenyl, alkoxy, alkanoyl or amido groups. An xe2x80x9caralkylxe2x80x9d group is an xe2x80x9calkylxe2x80x9d group substituted by an xe2x80x9carylxe2x80x9d group.
The additive composition of the present invention includes at least one compound of formula (I). It is preferred that none of R1, R2, R3 or R4 is hydrogen, i.e., four alkyl, alkenyl, aryl or aralkyl groups are present; in the case where the groups are alkyl, the compound of formula (I) is a tetraalkylimidazolidinethione (TAIT). The compound in which R1, R2, R3 and R4 are all methyl, i.e., tetramethylimidazolidinethione is also known as TMIT, and has the following structure: 
In one aspect of this invention, the additive composition contains a mixture of TAIT compounds produced from a mixture of ketones. In another aspect of this invention, the composition contains a mixture of tetrasubstituted compounds and compounds having less than four alkyl groups, i.e., those in which one or more of the substituents is hydrogen; such a mixture would be produced from a mixture of ketones and aldehydes, or from a mixture of aldehydes.
It is preferred that the group R in the acid, RCO2H, has from five to 17 carbon atoms. When R is alkyl or alkenyl, it is further preferred that R has from nine to 17 carbon atoms.
In the lubricant additive composition, the molar ratio of the compound of formula (I) to the acid RCO2H is preferably in the range from 3:1 to 1:5, and most preferably, from 2:1 to 1:2. For the purpose of calculating molar ratios in a case where multiple compounds of formula (I) are present, an approximate molecular weight is determined for the mixture using NMR spectral analysis of the product mixture, and averaging the molecular weights of the two predominant products.
In one aspect of this invention, the lubricant additive composition is added to a lubricating oil in an amount from 0.1% to 20% by weight. It is preferred that the additive composition is added in an amount from 0.1% to 10%, more preferably from 0.5% to 10%, and most preferably from 0.5% to 3%. A lubricating oil is a natural or synthetic oil, having suitable viscosity for use as a lubricant, or a mixture thereof.